Field
The present invention relates to a tire for a heavy vehicle designed to run on various types of ground, and in particular the tread of such a tire.
Although not limited to this application, the invention will be more particularly described with reference to a multipurpose agricultural machine that can run just as well in the fields as on the road, such as an agricultural tractor.
Description of Related Art
A tire for an agricultural tractor is designed to run on various types of ground such as the more or less compact earth of fields, the unpaved tracks for access to the fields and the paved surfaces of roads. Because of the diversity of use, in the field and on the road, a tire for an agricultural tractor, and in particular its tread designed to come into contact with the ground, must have a performance compromise between field traction, resistance to tearing, resistance to wear on the road, rolling resistance, vibratory comfort on the road.
The trend of enlargement in the size of agricultural operations is leading to increasingly frequent use of agricultural tractors on the road and to an increase in their speed in road usage up to speeds that can achieve 40 km/h or even 65 km/h. This evolution in usage involves ever greater demands with respect to tire performance on the road such as wear resistance, rolling resistance and vibratory comfort. Wear resistance, under high engine torque, of a tire for a drive axle of an agricultural tractor is currently a demanding performance, considering the increase in mechanical power of agricultural tractors up to, for example, values of the order of 500 hp.
In general, a tire has a geometry of revolution about a rotation axis, and the geometry of the tire may be described in a meridian plane containing the rotation axis of the tire. For a given meridian plane, the radial, axial and circumferential directions designate respectively the directions perpendicular to the rotation axis of the tire, parallel to the rotation axis of the tire and perpendicular to the meridian plane. In the following, the expressions “radially inner” and “radially outer” mean respectively “closer to the rotation axis of the tire, in the radial direction” and “further from the rotation axis of the tire, in the radial direction”. The expressions “axially inner” and “axially outer” mean respectively “closer to the equatorial plane, in the axial direction” and “further from the equatorial plane, in the axial direction”, the equatorial plane being the plane perpendicular to the rotation axis of the tire and passing through the middle of the tread surface of the tire.
In general, a tire comprises a crown comprising a tread designed to come into contact with the ground via a tread surface, two beads designed to come into contact with a rim and two sidewalls connecting the crown to the beads. A tire for an agricultural tractor, comprises a carcass reinforcement, anchored in each bead, usually consisting of at least one layer of textile reinforcing elements, the said reinforcing elements being substantially parallel with one another in the layer and being able to be substantially radial, that is to say substantially perpendicular to the circumferential direction, and/or crossed from one layer to the next while forming equal or different angles with the circumferential direction. The carcass reinforcement is usually surmounted radially externally by a crown reinforcement consisting of at least two working crown layers consisting of textile or metal reinforcing elements crossed from one layer to the next while forming slight angles with the circumferential direction.
The tread, during the rotation of the tire, comes into contact with the ground via a tread surface. The tread surface comprises two axial ends which are the axially outermost points of the said surface coming into contact with the ground. The axial distance, measured parallel to the rotation axis of the tire between the two axial ends of the tread surface, defines the tread width.
The tread generally comprises a plurality of protruding elements or lugs, of elongate parallelepipedal shape consisting of at least one rectilinear or curvilinear portion, and separated by furrows.
In the radial direction, a lug extends between an inner surface of revolution about the rotation axis of the tire and the tread surface, the radial distance between the inner surface and the tread surface defining the lug height. The radially outer face of the lug, belonging to the tread surface, which comes into contact with the ground when the lug passes in the area of contact of the tire, is called the contact face of the lug.
In the axial direction, a lug extends between an axially outer end face and an axially inner end face.
In the circumferential direction, a lug extends between a leading lateral face and a trailing lateral face. The leading lateral face is the face of which the radially outer ridge, the intersection of the said face with the tread surface, or leading edge is the first to come into contact with the ground when the lug enters the contact area of the tire with the ground, during the rotation of the tire. The trailing lateral face is the face of which the radially outer ridge, the intersection of the said face with the tread surface, or trailing edge is the last to come into contact with the ground when the lug enters the area of contact of the tire with the ground during the rotation of the tire.
A lug usually has an average angle of inclination relative to the circumferential direction, close to 45°. The average angle of inclination is the angle of the straight line passing through the respectively axially outer and inner ends of the centre line of the contact face, the centre line being all of the points of the contact face that are equidistant from the leading and trailing edges.
The tread of a tire for an agricultural tractor usually comprises two rows of lugs as described above. This distribution of lugs inclined relative to the circumferential direction confers on the tread a V shape normally called a chevron pattern. The two rows of lugs have a symmetry relative to the equatorial plane of the tire, most frequently with a circumferential offset between the two rows of lugs, resulting from a rotation about the axis of the tire of one half of the tread relative to the other half of the tread. Moreover, the lugs may be continuous or discontinuous, and distributed circumferentially with a constant or variable pitch.
Document U.S. Pat. No. 4,383,567 describes a tread for a tire of an agricultural tractor, comprising an alternation of long lugs and short lugs. According to this document, a long lug, of which the axial distance between ends is greater than half the axial width of the tread, consists of three rectilinear portions, whereas a short lug, of which the axial distance between ends is less than half the axial width of the tread, consists of two rectilinear portions. Document U.S. Pat. No. 4,534,392 proposes a variant of the foregoing solution in which a combination of two long lugs in succession alternates with one short lug. The advantage of such treads is to improve comfort of running on the road without reducing the traction performance in the field. However, these solutions have the drawback of causing more rapid wear of the short lugs relative to the long lugs, leading to irregular wear of the tread and the premature removal of the tire.
Documents U.S. Pat. No. 5,046,541 and U.S. Pat. No. 5,411,067 describe respectively discontinuous short lugs and long lugs which substantially improve the problem of irregular wear mentioned above. The drawback of these solutions is that they lead to more rapid wear of the tire, which is a cause of premature removal of the tire.
Document U.S. Pat. No. 6,382,284 aims to improve the compromise between traction and wear by proposing a dissymmetrical tread, with short lugs at the shoulder and protruding elements at the centre performing the function of the long lugs of the solutions previously described. The dissymmetry of the design has the drawback of a fitting constraint between the left and right sides of the axle of the agricultural tractor. The speed of wear at the centre of the tread is also increased because of the dissymmetry of the design, hence a reduction in the period of use of the tire.